The invention concerns the transmission of video signals in confined spaces such as tunnels and mines.
Video transmission is used in systems for remote surveillance of the environment or of a mobile plant moving in an ill-defined environment subject to interference, with the aim of viewing and guiding or controlling mobile plants; one particular example is the high data rate, analog transmission of information and telecontrol signals to a machine in a mine tunnel, a drainage tunnel or, more generally, any tunnel with a diameter in the order of 1 to 10 meters.
Until now video signals have been transmitted in tunnels for broadband communications between the mobile plant and a fixed (or at least temporarily fixed) equipment in the tunnel using radiating structures; these might be coaxial cables operating in the 200-900 MHz frequency band, for example, or microwave waveguides radiating frequencies above 1 GHz. In this way it is possible to secure continuous transmission between a mobile transmitter moving near the waveguide and a receiver at the end of the waveguide. A communication system of this kind is bidirectional as the waveguide can also be used for transmission from the fixed equipment carrying the receiver to the mobile plant carrying the transmitter.
These known systems have the disadvantage that the mobile plant is required to remain in the immediate vicinity of the waveguide (within about 1 meter) if the benefits of the homogeneity of the field are to be obtained. More importantly, these systems have the major disadvantage of requiring the installation of radiating strutures representing a costly infrastructure. The transmission system is, therefore, a fixed system devoid of any flexibility for future adaptation.
The invention is directed to alleviating these disadvantages by providing a microwave video transmission system establishing continuous communication with mobile plants at any position in a tunnel, the communication quality being good at all times in spite of fluctuations in the received signal resulting from the numerous reflections from the tunnel walls and from the usual presence of metal obstacles between the transmitter and the receiver and attenuation due to bends or branches in the tunnel.